1. Field of the Invention
The present invention relates generally to a capacitive fill level measurement device with a fill level sensor which has several sensor fields, with electrical wires connected to the sensor fields of the fill level sensor, with a selector switch connected by its multipole side to ends of the wires away from the sensor fields of the fill level sensor, and with a power supply and evaluation circuit which is connected to the monopole side of the selector switch, the sensor fields of the fill level sensor being made planiform and located on top of one another, and consisting preferably of metal.
2. Description of the Related Art
Capacitive fill level measurement devices are used for capacitive acquisition of the fill level of liquids, bulk materials and other loads in a closed or an open container and are common (compare, for example, German patent publication DE 196 44 777, European patent disclosure documents 0 916 930, 0 926 474, French patent disclosure document FR 2 662 249, U.S. Pat. Nos. 3,935,739, 4,350,039, 4,350,040, 4,589,077, 4,780,705, 5,142,909 and 5,406,843).
In capacitive fill level measurement devices, in terms of measurement engineering, the fact is often used that the load with a fill level which is to be determined influences the capacitance between the fill level sensor or between a sensor field of the fill level sensor on and a conventional reference electrode. This is because the dielectric constant of the load, which is also important for the capacitance between the fill level sensor or between one sensor field of the fill level sensor and the reference electrode differs from the dielectric constant of air. Consequently, as described above, capacitive fill level measurement devices of the type under consideration here as dictated by the needs of operation include a fill level sensor which has several sensor fields, a power supply and evaluation circuit.
As necessary, the sensor fields of the fill level sensor can be connected to the power supply and the evaluation circuit. This is done by the electrical wires which are connected to the sensor fields of the fill level sensor and the selector switch, which is connected by its multipole side to the ends of the wires away from the sensor fields of the fill level sensor, and the power supply and evaluation circuit is connected to its monopole side. The selector switch, therefore, as dictated by the requirements of operation on the one side, called the multipole side, has a plurality of terminals, while on the other side, called the monopole side, there is only one terminal; the selector switch therefore makes it possible to electrically connect one selectable terminal of the multipole side to the terminal of the monopole side or vice versa.
In examining the power supply and evaluation circuit in functional terms, it includes a power supply circuit and an evaluation circuit; while the power supply circuit is used to make available the necessary power supply voltage and the necessary power supply current to the fill level sensor, using the evaluation circuit the fill level of the load is determined, therefore, it is determined which sensor field of the fill level sensor has already been or is no longer being or has not yet reached by the load. With reference to the selector switch which can be made, for example, as a multiplexer (compare German patent publication DE 196 44 777) it can be stated that for the power supply function the power supply and evaluation circuit can be connected from the terminal of the monopole side to the terminal of the multipole side, while for the evaluation function the sensor field of the fill level sensor can be connected via the terminal of the multipole side and the terminal of the monopole side to the power supply and evaluation circuit.
It was stated initially that in the capacitive fill level measurement device underlying the invention (compare German patent publication DE 196 44 777), the sensor fields of the fill level sensor are made planiform and are located on top of one another and consist preferably of metal. Here planiform does not mean that the sensor field should be made essentially only two-dimensionally, but rather, only that the two-dimensional execution is significant, because for determining the fill level a change of the capacitance between a sensor field which acts as the measurement electrode and a conventional reference electrode is significant. The sensor elements are located on top of one another with respect to the fact that the fill level of a load located in a vessel is to be determined means only that the sensor fields is their geometrical extension are in any case also located on top of one another. Accordingly, the sensor fields can also be located next to one another and also overlapping on top of and/or next to one another. Ultimately, the sensor fields of the fill level sensor need not consist of metal; what is important with respect to material is in turn that each sensor field must be suitable as a measurement electrode, therefore must be suitable for accomplishing a capacitance which is changed by a changing dielectric constant, together with only one electrode.
An object of the invention is to devise a capacitive fill level measurement device of the type under consideration and described above, which can be manufactured with simple production technology and economically for a host of different applications and/or which largely meets the requirements of today in terms of electricity.
The capacitive fill level measurement device in accordance with the present invention is characterized by either electrical-mechanical, electrical-construction, or electrical-geometrical measures, and, in addition to the sensor fields which are located on top of one another, on one side or on both sides there includes a matrix of printed conductors which run vertically and horizontally, each horizontally running printed conductor on one side is connected to one sensor field and on the other side to a vertically running printed conductor, and each horizontally running printed conductor with the vertically running printed conductor which is connected to it forms an electrical line or a part of an electrical line.
It is important for the capacitive fill level measurement device in accordance with the present invention has a fill level sensor which is made such that the individual sensor fields are implemented, activated or connected to the electrical wires, or are provided with electrical wires in manner which is advantageous over conventional prior art devices. The special fill level sensor built in accordance with the present invention consists mainly in that the matrix provided next to the sensor fields including horizontally and vertically running printed conductors makes it possible to use a starting material for the fill level sensor which can be used for a host of different applications, in particular, different fill level measurement devices.
One such starting material which includes a conductor support, made preferably planiform, of a plurality of sensor fields which are made on top of one another in one plane parallel to the plane of the conductor support, which are made planiform and which are provided on the conductor support. and a matrix which is provided next to the sensor fields, on one side or on both sides, consisting of horizontally and vertically running printed conductors, all horizontal printed conductors with all vertical printed conductors being connected to one another so that all horizontal printed conductors and all vertical printed conductors are electrically connected to one another.
In the above described starting material for a fill level sensor, has several sensor fields, therefore, the sensor fields, horizontal printed conductors and vertical printed conductors are electrically connected to one another. In addition, the sensor fields, horizontal printed conductors and vertical printed conductors are at the same potential, this, of course, is not functionally compatible for the later use of a specific fill level sensor of a specific capacitive fill level measurement device. Consequently, the above described starting material must be processed in a special way for the fill level sensor of a capacitive fill level measurement device, which sensor has several sensor fields. Therefore, a special process is necessary for producing a fill level sensor which has several sensor fields and which can be used for a capacitive fill level measurement device from the above described starting material. This process is characterized in that, except for the horizontal printed conductors and the vertical printed conductors which are to be connected to one another as necessary for operation, all other horizontal printed conductors and vertical printed conductors are separated from one another such that only the horizontal printed conductors and the vertical printed conductors which are to be connected to one another as is necessary for operation are in fact connected to one another.
The capacitive fill level measurement device in accordance with the present invention is characterized in a second embodiment by electrical-functional and electronic-functional measures, whereby the power supply and evaluation circuit delivers a high frequency power supply voltage, the power supply voltage is frequency-spread by a noise signal originating from a noise source. The frequency-spread power supply voltage leads to at least one of a frequency-spread measurement quantity, a measurement voltage or a measurement current. The frequency-spread power supply voltage is placed at the first input of the correlator and the frequency-spread measurement quantity is placed at the second input of the correlator and the output signal of the correlator is supplied to the other evaluation circuit.
The capacitive fill level measurement device in accordance with the present invention is a so-called open electronic system, i.e., the sensor surfaces of the fill level sensor cannot be entirely shielded, and thus, emit electromagnetic radiation and signals into the environment and vice versa absorb electromagnetic radiation and signals from the environment. This circumstance can lead to noise emissions of the capacitive fill level measurement device and also lead to incident noise emissions. Accordingly, this problem is important in capacitive fill level measurement devices of the type under consideration because the distance between the sensor elements of the fill level sensor and the power supply and evaluation circuit can be considerable.
By means of the above described electrical-functional and electronic-functional measures, a capacitive fill level measurement device is formed in which the noise emission and the sensitivity to incident noise emissions are relatively low. The frequency spreading of the power supply voltage which takes place by means of a noise signal reduces the bandwidth and thus the spectral energy density of the power supply voltage on the one hand and the measurement quantity on the other without needing to reduce the power supply voltage on the sensor elements. In this way, the amplitude of the emitted noise signals is reduced if the maximum allowable amplitude of the noise signals emitted into the capacitive fill level measurement device in accordance with the present invention is increased. Thus, both the disruption of other devices by the capacitive fill level measurement device in accordance with the present invention as well as the sensitivity of the fill level measurement device to noise from by other devices are reduced.
With respect to what can be achieved in particular by the frequency spreading of the power supply voltage and along with this by the frequency spreading of the measurement quantity and how the teaching xe2x80x9cfrequency spreadingxe2x80x9d can be implemented in particular, in order to avoid repetitions reference is made to all the disclosure contents of German patent disclosure document 198 13 013, reference being made expressly for the disclosure content of this patent application.
Finally, the capacitive fill level measurement device in accordance with the present invention in a third embodiment is characterized by other electrical-functional and electronic-functional measures, a electrically conductive shield is assigned to the sensor fields and/or the wires and/or the selector switch whereby the electrically conductive shield is always at a potential which corresponds essentially to the potential of the sensor fields, the wires and the selector switch. The above described measure of assigning the electrically conductive shield to the sensor fields and/or the wires and/or the selector switch is used for the same purposes for which the teaching xe2x80x9cfrequency spreadingxe2x80x9d is also used. Specifically, to reduce the amplitude of the emitted noise signals, and to increase the maximum allowable amplitude of the noise signal emitted into the capacitive fill level measurement device in accordance with the present invention.
The aforementioned teaching xe2x80x9cshieldxe2x80x9d is associated with the problem that the shield represents a capacitive load of the sensor fields and/or the wires and/or the selector switch. This problem is eliminated when, as provided in accordance with the present invention, the electrically conductive shield is always at a potential which essentially corresponds to the potential of the sensor fields, the wires and the selector switch. If at no time there is a potential difference between the electrically conductive shield and the sensor fields or the wires and the selector switch, a current which represents a capacitive load cannot flow, regardless of how large the capacitance is between the electrically conductive shield and the sensor fields or the wires and the selector switch.
The measure of providing for the electrically conductive shield to always be at a potential which essentially corresponds to the potential of the sensor fields, the wires and the selector switch can in particular be implemented by the potential of the electrically conductive shield being obtained from the potential of the sensor fields, the wires and the selector switch via the control of the potential of the electrically conductive shield. But this approach has the disadvantage that a control deviation is always necessary and that special problems occur in dynamic processes.
With reference to the measure xe2x80x9cpotential equalityxe2x80x9d which was treated individually above, another teaching of the invention which acquires special importance states that the electrically conductive shield is connected via a current measurement circuit to the sensor fields, the wires and the selector switch and the current measurement circuit has an essentially negligibly small internal resistance. This current measurement circuit can includes a synchronous rectifier, a lowpass connected downstream of the synchronous rectifier, and a current-voltage converter which is connected downstream of the lowpass. In this current measurement circuit the synchronous rectifier and the downstream lowpass lead to the fact that a direct current is formed from the high frequency measurement current which is supplied on the inlet side and from it then a dc voltage is formed by the current-voltage converter.
In the prior art, it is quite generally common to implement voltages as a potential difference to the frame potential or to the ground potential, i.e., that one output of a corresponding voltage source xe2x80x9cis highxe2x80x9d, while the other output is connected to the frame potential or the ground potential. In the past, xe2x80x9cframe potential or ground potentialxe2x80x9d was always stated carefully, because often, if not quite ordinarily, the frame potential and the ground potential are the same. In a preferred embodiment of the capacitive fill level measurement device in accordance with the present invention, there is a difference which is characterized in that the power supply voltage which is made available by the power supply and evaluation circuit is between the ground potential and the frame potential, thereby a xe2x80x9cfloatingxe2x80x9d frame potential is accomplished.
In doing so, of course, provisions must be made for the frame potential and the ground potential not be connected to one another without impedance. Consequently, it is recommended that the power supply circuit of the power supply and evaluation circuit be connected dc-decoupled to the ground potential, for example, by a decoupling capacitor. On the other hand, the evaluation circuit of the power supply and evaluation circuit is connected ac-decoupled to the ground potential, for example, by at least one current-compensated interaction limiting reactor, preferably, by several current-compensated interaction limiting reactors.
An object of the invention is to devise a capacitive fill level measurement device which can be economically built for a host of different applications. This means that the fill level measurement device in accordance with the present invention can be used for acquiring or determining the fill level of loads with quite different dielectric constants and for acquiring or determining the fill level of loads in quite different containers. To accomplish this, preferred embodiments of the fill level measurement device in accordance with the present invention are characterized in that the frequency of the power supply voltage can be controlled, for example, by a microprocessor and/or the gain of the evaluation circuit of the power supply and evaluation circuit can be controlled, preferably, in turn by a microprocessor. These measures, used alternatively or cumulatively, make it possible to consider the very different dielectric constants of the loads and very different embodiments, especially very different geometries of the containers which hold the loads. This is done with the objective of arriving at measurement results which are as accurate as possible and which can be processed as easily as possible, especially with the objective of optimally using the possibilities of the evaluation circuit of the power supply and evaluation circuit, specifically using the existing control range without overload occurring.
As was stated above, one preferred embodiment of the fill level measurement device in accordance with the present invention is characterized in that a electrically conductive shield is assigned to the sensor fields and/or the wires and/or the selector switch and that the electrically conductive shield is always at a potential which corresponds essentially to the potential of the sensor fields, the wires and the selector switch. In this embodiment therefore the electrically conductive shield is not at a constant potential, specifically the ground potential; rather radiation and signals are easily emitted into the environment via this electrically conductive shield and this electrically conductive shield is easily able to absorb radiation and signals from the environment. To prevent this, another teaching of the invention is to provide a second electrically conductive shield which is used to electrically conductive shield the first electrically conductive shield, the second electrically conductive shield preferably being at the ground potential. Via the second electrically conductive shield the first electrically conductive shield, and thus, the sensor fields, the wires, and the selector switch are shielded xe2x80x9cin the classical sensexe2x80x9d.
In the above described embodiment of a capacitive fill level measurement device having a second electrically conductive shield which is at ground potential, the second electrically conductive shield, of course, represents a capacitive load of the first electrically conductive shield, therefore, a current flows from the first electrically conductive shield to the second electrically conductive shield and vice versa. But this does not affect the measurement results when, as is preferably provided, the first electrically conductive shield is connected via a current measurement circuit to the sensor fields, the wires, and the selector switch and the current measurement circuit has an essentially negligibly small internal resistance. Specifically, the measurement current flowing via the current measurement circuit is not influenced by the current which results from the capacitive loading of the first electrically conductive shield by the second electrically conductive shield.